1. Field of the Invention
This invention relates to a strain gauge, whose position and orientation on the surface of an object can be detected in an optical manner. The invention also relates to a system and method for spatially locating such gauges placed on the surface of an object such as an aircraft structure.
2. Description of the Related Art
Structural tests are known, performed on complete aircraft to validate the predictive calculations made upstream by computerised modelling, of the structural performance of new aeronautical structures (aircraft wings, etc.). These predictive calculations enable manufacturers to make very early strategic decisions on the products being developed.
These structural tests typically involve two types of test: static load tests and dynamic fatigue tests. During these tests, actuators develop loads that are applied to the aircraft structure, for example in order to recreate certain loads encountered in flight.
The behaviour of the structure in response to these stresses is therefore monitored in a permanent manner by means of sensors positioned on the surface of the structure being tested.
These sensors are strain gauges, also known as stress gauges, each measuring the local deformation suffered by the structure.
Each of these strain gauges therefore comprises an element to be reversibly lengthened by means of a force applied while displaying a variation in the resistance thereof. This lengthening takes place along an axis defining the measurement axis of the gauge. By measuring this very low variation in resistance, the deformations suffered in the area of measurement of the gauge by the structure being tested can be traced.
However, a multitude of gauges, typically more than 1,000, are required to test large-scale structures, which involves the need to resort to multi-channel and specialised data acquisition systems.
Given that each of these gauges is manually assembled onto the structure by qualified operators, this assembly operation is particularly laborious, all the more so as each gauge must be wired and calibrated.
Each gauge must be placed in a precisely defined position on the structure so as to correspond to the calculations made and be individually identified in order to associate a measurement with a precise pinpoint area of the real structure being tested.
However, inaccuracies can be observed in the positioning of certain gauges, capable of resulting, for example, in an inaccuracy in the angular orientation of the measurement axis of these gauges, or even in their real location in relation to their theoretical position.
These inaccuracies lead to residual deviations thus distorting the comparison made between the strength of the structural parts calculated and of the real structural parts.
Therefore, it could be highly beneficial to use a system that can easily determine the exact position and orientation of the gauges on the real structure in order to allow for a more exact comparison to be made between the behaviour of the real structure and the values estimated by computer simulation.